House Republicans for the first time Wednesday linked new IRS rules designed to clamp down on nonprofits with embattled former IRS employee Lois G. Lerner, saying her involvement taints the entire process and should force the tax agency to cancel its crackdown.

Republicans argue that the rules, proposed in November, are designed to silence interest groups’ free speech — which they say was the same reason the IRS asked intrusive questions and delayed tea party groups’ applications for tax-exempt status over the past few years.

Ms. Lerner, a former head of the division that reviewed those applications, was included on an email in 2012 in which staffers hinted at a secret effort to write the crackdown rules.

The email was released Wednesday by the House Ways and Means Committee, which said it shows Ms. Lerner’s involvement and proves that the rules were in the works for years, not a response to the scandal revealed in June. Republicans said that taints the whole process.

“If Treasury and the IRS fabricated the rationale for a rule change it would tend to raise questions about the integrity of the rule-making process,” said Rep. Dave Camp, Michigan Republican and the committee chairman, who has called on the IRS to cancel the rules change.

Democrats countered that Republicans haven’t proved that the IRS targeted anyone for political reasons and questioned why they are pursuing the investigation since no smoking gun has been found.

“It’s clear you’re trying to keep this issue alive for political purposes,” said Rep. Sander M. Levin of Michigan, the ranking Democrat on the committee.

The fight has spilled well beyond the boundaries of the IRS. Republicans also have called into question the Justice Department’s criminal probe of the tax agency, arguing that the investigation is tainted by political donations to President Obama made by one of the lawyers in the investigation, and by Mr. Obama’s comments this weekend that seemed to prejudge the outcome, saying there was no corruption.